The present invention relates to a tracking error signal correcting system for a writable optical disc, and more particularly to a system wherein an accurate tracking error signal is always obtained, before and after information is written on the disc.
There are optical discs such as a CD including a read only CD and a writable disc which is used as a ROM. Further, as a writable optical disc having a high recording density, a write once disc and an erasable disc are provided. Information is recorded on the disc and reproduced with a laser beam. These discs are different from the CD in the material of the recording surface.
For example, the write once (CD-WO) disc has a tellurium or bismuth recording surface on which the lasers burn pits for recording. In another type of the CD-WO discs, the lasers are focused on a recording surface coated with a selenium antimony (Sb.sub.2 Se.sub.3) thin film, or an oxide tellurium (TeOx) thin film, or a thin film of organic pigment, changing the reflectivity of the light.
The erasable disc uses as the recording surface, an amorphous alloy made of rare earth metals such as gallium, terbium, and others. In a magneto-optical recording method, the recording surface of the disc is initially magnetized to form a magnetic field in a direction perpendicular to the surface. The laser heats a predetermined area of the disc to elevate the temperature above Curie temperature, which is about 150.degree. C., thereby reversing the direction of the magnetic field. To read the recorded information, the laser is irradiated on the recording surface so that polarized wave front slightly rotates as a result of the Kerr effect. Thus only the polarized wave deflected by the rotation is read by a photodetector, thereby enabling to read the information.
Referring to FIG. 5, an optical pickup for writing on and reading from these optical discs has a laser diode 1 which emits a laser beam. The laser beam is focused on the recording surface of the optical disc through a collimator lens 2, multi-prism 3, reflective mirror 4, and an objective 5. A part of the beam reflected from the optical disc passes through the multi-prism 3 and is received by a photodetector 6 so as to control the intensity of the laser beam through a feedback circuit. The other part of the reflected beam is transmitted through a convex lens 7, cylindrical lens 8, and a polarizing beam splitter 9 to a photodetector A for producing a tracking error signal, and to a photodetector B for producing a focus error signal.
In order to record information on the optical disc, an intense laser beam is emitted from the laser diode 1. In order to read information recorded on the disc, a laser beam with less intensity is needed. Hence, gains of the circuits for generating the focus error and tracking error change dependent on the intensity of the laser beam. As a result, servo systems cannot accurately operate.
FIGS. 6a and 6b show automatic gain control system for solving such a problem. Referring to FIG. 6a, the photodetector A has four detector elements A1, A2, A3 and A4. The outputs of the parallely aligned detector elements A1 and A3 are applied to a non-inverting terminal of a subtractor 10 and the output of the remaining detector elements A2 and A4 are applied to an inverting terminal of the subtractor 10 to generate an output Xa corresponding to a tracking error. Outputs of all of the detector elements A1 to A4 are applied to an adder 11 to produce an output Ya corresponding to the quantity of light of the laser beam. The outputs Xa and Ya are fed to a divider 12, where a ratio Xa/Ya is calculated to produce a tracking error signal TE.
Similarly, as shown in FIG. 6b, the photodetector B has four detector elements B1, B2, B3 and B4. The outputs of the diagonal detector elements B1 and B4 are fed to a non-inverting terminal of a subtractor 13. The outputs of the detector elements B2 and B3 are fed to an inverting terminal of the subtractor 13. Hence, the subtractor 13 produces an output Xb which corresponds to a focus error. The output of the detector elements B1 to B4 are fed to an adder 14 which generates an output Yb corresponding to the quantity of light of the laser beam. A ratio Xb/Yb is obtained at a divider 15, thereby producing a focusing error signal FE.
The ratio of the tracking error and the focusing error to the whole quantity of light at the recording is equal to the ratio at the reproducing of the recorded information. Thus, the servo systems can be accurately operated.
However, the above-described system is effective only when used for a magneto-optical discs, where the modulation rate of the tracking error signals produced by a push-pull method after a recording does not largely change from that produced before the recording. To the contrary, in a disc covered with a pigment film (CD-WO), or a phase change disc used as an erasable disc, the modulation rate changes so much that the gain of the servo system varies and hence the tracking error signal becomes unstable. That is to say, due to various shapes and depths of the pits formed at the recording, the quantity of the tracking error does not change in proportion to the quantity of light.